Silk is a natural protein fiber, some forms of which can be woven or knitted into textiles. The protein fiber of silk is composed mainly of fibroin (75-80%) and a smaller coating of sericin (20-25%). It is most commonly produced by certain insect larvae to form cocoons. The best-known silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori, which are reared in captivity (sericulture). The shimmery appearance of silk is due to the triangular prism-like structure of the silk fiber, which causes silk cloth to refract incoming light at different angles, thus producing different colors.